Torque motor driven spool valves are well known in the art including such valves which operate through the utilization of a rotary torque motor having a drive member extending from the rotor thereof into contact with the spool valve to directly reciprocate the spool valve within a bore provided in the valve housing. Typically the spool valve is constructed of 440c stainless steel and the drive member is tungsten carbide. When the spool valve reciprocates it controls the flow of fluid from a source thereof to a load in response to the electrical signals applied to the drive motor.
Direct drive servovalves of the type above mentioned are illustrated in the following U.S. Pat. Nos. 2,697,016, 2,769,943, 3,550,631, 4,339,737, 4,197,474, 4,452,423, 4,641,812, 4,645,178, 4,793,337, 5,052,441 and 5,040,568.
In all such direct drive servovalves the spool valve is reciprocated by the free end of the motor shaft contacting the spool through an eccentrically mounted pin having a substantially spherical drive tip. The drive tip may be formed with flat surfaces thereon if desired. The drive tip is inserted into a well or annular groove formed in the spool. The dimensional relationship between the spherical drive tip and the spool is such as to provide minimal frictional forces and near zero backlash. Utilizing such dimensions necessitates lapping and fitting operations which add greatly to the expense of such devices.
As one means of simplifying the construction and operation of such valves, motor to spool couplings as illustrated and described in U.S. Pat. Nos. 5,263,860 and 5,263,861 were made. U.S. Pat. No. 5,263,860 discloses an intricately shaped coupling including a molded plastic member having three fingers which engage a pin extending from the motor shaft. The pin is press fitted into engagement with the fingers and causes the fingers to outwardly expand. U.S. Pat. No. 5,263,861 discloses a brass two piece bushing having an "0" ring encircling it. A pin extending from the motor shaft is inserted into the bushing causing the two halves to separate slightly against the compression force of the "0" ring. Each of these structures operate excellently for the purpose intended but are still some what complex and costly to manufacture.